Method for rapid screening of emission-mix using a combinatorial chemistry approach

ABSTRACT

A method of making an electron emissive material using combinatorial chemistry techniques is provided. The method includes providing a plurality of pixels of the electron emissive material, each pixel having at least one different characteristic from any other one of the plurality of pixels, and measuring at least one property of each pixel. The measurement may include a measurement of the electron emissive material work function using a Kelvin probe or other work function measurement systems.

BACKGROUND OF THE INVENTION

[0001] This invention generally relates to a method for preparingceramics and more particularly to a combinatorial method for preparingelectron emissive ceramic materials for lamp cathodes.

[0002] The standard emissive coating currently used on a majority ofcathodes of commercial fluorescent lamps contains a mixture of barium,calcium, and strontium oxides (“the triple oxide emissive mixture”).Because these oxides are highly sensitive to CO₂ and water, they areplaced on the lamp cathodes initially as a mixture of barium, calciumand strontium carbonates in a slurry suspension containing a binder anda solvent. The mixture of carbonates is then “activated” during themanufacturing process by resistively heating the cathodes until thecarbonates decompose, releasing CO₂ and some CO, and leaving behind thetriple oxide emissive mixture on the lamp electrode. It is believed thatbarium in barium oxide, in some form, is primarily responsible for theelectron emission. It is also known to add a small amount of Al, Hf, Zr,Ta, W and Th dopants to the triple oxide emissive mixture, as discussedin U.S. Pat. No. 3,563,797 to Young, incorporated herein by reference inits entirety.

[0003] The triple oxide emissive mixture suffers from severaldisadvantages. Lamps having cathodes coated with the triple oxideemissive mixture have a higher than desired work function, which leadsto a higher than desirable cathode fall voltage.

[0004] Other emissive materials for fluorescent lamps are also known.For example, U.S. Pat. No. 4,319,158 to Watanabe, incorporated herein byreference in its entirety, discloses an emissive material whichcomprises yttrium oxide and lanthanum oxide. This emissive material maybe used in combination with the triple oxide or barium tungstateemissive materials. Furthermore, it has been previously suggested inU.S. Pat. No. 4,031,426 to Kern, incorporated herein by reference in itsentirety, to substitute the triple oxide emissive mixture with bariumtantalate emissive materials having various barium to tantalum ratios.While the materials of Watanabe and Kern have longer lifetimes than thetriple oxide material, they have a lower efficacy than the triple oxide.

[0005] The work function of the emissive material depends on a varietyof factors, such as activation schedule, morphology, composition andstoichiometry, among others. Therefore, a variety of various emissivematerials have been prepared and screened in order to obtain an emissivematerial with a low work function. The prior art methods prepare andmeasure the emissive material of a given composition and stoichiometryone sample at a time. Therefore, these prior art methods are slow andinefficient. The present invention is directed to overcoming or at leastreducing the effects of one or more problems set forth above.

BRIEF SUMMARY OF THE INVENTION

[0006] In accordance with one preferred aspect of the present invention,there is provided a method of making an electron emissive material,comprising providing a plurality of pixels of the electron emissivematerial, each pixel having at least one different characteristic fromany other one of the plurality of pixels, and measuring at least oneproperty of each pixel.

[0007] In accordance with another preferred aspect of the presentinvention, there is provided a method of determining a work function ofa plurality of pixels of an electron emissive material, comprisingproviding an array of pixels of a first material on a first substrate,each pixel having at least one different characteristic from any otherone of the plurality of pixels, and measuring the work function of eachpixel on the first substrate using a work function measurement device.

[0008] In accordance with another preferred aspect of the presentinvention, there is provided a Kelvin probe combinatorial testingsystem, comprising a Kelvin probe apparatus, a first substrate adaptedto support a plurality of pixels of a material to be tested, each pixelhaving at least one different characteristic from any other one of theplurality of pixels, and a computer electrically connected to the Kelvinprobe apparatus containing software which analyzes a work functionmeasured on the plurality of pixels and which provides a visual,electronic or printed output of the work function of each pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is three dimensional view of a pixel array according to oneaspect of the first preferred embodiment of the present invention.

[0010]FIG. 2 is a top view of a pixel array according to another aspectof the first preferred embodiment of the present invention.

[0011]FIG. 3 is a side cross sectional view along line A-A in FIG. 2.

[0012]FIGS. 4 and 5 are side cross sectional views of a pixel arrayaccording to another aspect of the first preferred embodiment of thepresent invention.

[0013]FIGS. 6 and 7 are a three dimensional and a side cross sectionalview, respectively, of a pixel with a well according to one aspect ofthe first preferred embodiment of the present invention.

[0014]FIG. 8 is a three dimensional view of a pixel array fabricationaccording to the second preferred embodiment of the present invention.

[0015]FIG. 9 is a three dimensional view of a Kelvin probe system forscreening a pixel array according to the preferred embodiments of thepresent invention.

[0016]FIG. 10 is a side cross sectional view of a multitip Kelvin probesystem for screening a pixel array according to the preferredembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present inventor has realized that a combinatorial chemistryapproach may be used to rapidly screen various electron emissivematerial compositions to select the composition with the optimum desiredproperty, such as the lowest work function. The preferred embodiments ofthe present invention provide methods for the preparation, measurementand use of emissive materials using combinatorial techniques.Preferably, the emissive material can be any material, which when coatedon an electrode, emits electrons in response to a current or voltageapplied to the electrode. Examples of emissive materials are the tripleoxide and barium tantalate having various dopants and stoichiometries.The emissive materials are preferably suitable for use on cathodes of afluorescent lamp. However, the emissive materials may be used in otherapplications, such as electron emitters for electron picture tubes, andas electron injectors for particle accelerators and as pulsed RFsources.

[0018] According to a first preferred embodiment of the presentinvention, a plurality of dopants of a different composition and/or of adifferent concentration are added to a host material. The dopants maycomprise any molecules, ions or compounds which can be reacted with thehost material. Once dopants have chemically reacted with the hostmaterial, an array of emissive materials is formed. The array is thentested for useful emissive properties, such as work function, efficacyand cathode fall voltage.

[0019] According to a second preferred embodiment of the presentinvention, an array of emissive material pixels, each having a differentcomposition and/or stoichiometry, is deposited on a single substrate.The composition and stoichiometry of each pixel of emissive material inthe array may be continuously varied by depositing the materials viaseveral independent particulate injectors onto a large substrate, suchas a conductive aluminum substrate. The deposited emissive materialarray can then be tested for useful emissive properties, such as workfunction, cathode fall voltage and/or efficacy in a fluorescent lamp.

[0020] The deposited ceramic emissive materials of the first and secondpreferred embodiment are preferably activated and/or sintered byheating, using heating sources such as RF or a resistive filamentheater. Once the emissive materials are formed, then the various pixelsof emissive material each having a unique composition and/orstoichiometry are analyzed for useful emissive properties. Preferably, ascanning Kelvin probe is used to measure a work function distributionamong the pixels. The pixels which contain an emissive material having adesired work function are then selected for further testing, such asplacing the emissive material of the pixel on an electrode of afluorescent lamp. However, the work function distribution measurement isnot limited to using a Kelvin probe. Any diagnostic system that canmeasure the work function distribution can be used instead, such as anauger spectroscopy, electrostatic force microscopy, atomic forcemicroscopy or scanning tunneling microscopy system.

[0021] According to a first preferred embodiment of the presentinvention, an array of emissive materials is prepared by successivelydelivering dopants of materials to predefined regions on a hostmaterial, and allowing the dopants and the host material to chemicallyreact to form at least two different emissive materials having differentdopants and/or different dopant concentration (i.e., a different finalcomposition and/or stoichiometry, respectively). In one embodiment, forexample, a first dopant is delivered to a first region on a hostmaterial, and a second dopant of a different type and/or concentrationis delivered to a second region on the host material. Optionallythereafter, additional dopants can be added to the first and secondregions of the host material. Furthermore, different dopants and/ordifferent dopant concentrations may be added to the third through Nthregions of the host material. In addition to the combinatoriallydelivered dopants, one or more dopants may already be present in theemissive host material.

[0022] The host material may be made by various ceramic processingmethods. In a preferred aspect of the first embodiment, the hostmaterial comprises the barium oxide—strontium oxide—calcium oxideemissive mixture (i.e., the triple oxide). The triple oxide hostemissive material is made by mixing the starting barium, strontium andcalcium carbonate powders in a desired stoichiometric proportion. Themixed powder is then heated to activate or convert the carbonate powdersto oxide powders and to form a solid sintered body or “cake.”Preferably, the sintering takes place at a temperature of about 1300 to1500° C. for about 1 to 10 hours. However, other appropriate sinteringtemperatures and durations may be used. However, other emissive hostmaterials, such as barium tantalate may be used instead.

[0023] The dopants in the individual reaction regions of the hostmaterial are preferably prevented from moving to adjacent reactionregions. This may be accomplished by fabricating the host material intoan array of pixels. There may be any desired number of pixels, such asfrom 2 to 10,000 pixels, preferably from 10 to 1000 pixels.

[0024] In a first preferred aspect of the first embodiment, a hostmaterial 1 is fabricated into a block and then diced into individualpixels 2 or sections, as in FIG. 1. Dicing may be carried out to cutpartially into or all the way through the host material 1. Pixel 2 inthis instance means a pillar-like protrusion over a surface of the hostmaterial 1. Each pixel can be of any shape, such as circular orpolygonal, or of various dimensions, as long as the pixel is capable ofreceiving discrete amounts of dopant. Pixels preferably are over about10 microns in thickness, and in one preferred embodiment have a heightof between about 100 microns to about 5 mm. Individual pixels for dopingare connected to a base 1 of the host material.

[0025] Alternatively, the emissive host material may be formed bycasting or molding the mixed carbonate powder into a mold cavity or intoa recesses on a substrate having an appropriate shape. For example, in asecond preferred aspect of the first embodiment, a suspension of abinder, solvent and the mixed barium, strontium and calcium carbonatepowders are placed or cast into recesses 5 in a forming substrate 6, asillustrated in FIGS. 2 and 3. The binder may comprise organic material,such as nitrocellulose, and the solvent may comprise butyl acetate, amylacetate, methyl propanol, or the like. The suspension is then heattreated to evaporate the binder and to activate and sinter the carbonatepowders to form the triple oxide emissive host material I pixels 2 inthe recesses 5 of the substrate 6. The substrate 6 may comprise anymaterial that is capable of withstanding the activation temperature. Thedopants different type and/or concentration are then added to the hostmaterial pixels 2 in each recess 5 and reacted to form doped pixels 2.The doped pixels 2 located in the recesses 5 of the substrate 6 are thensubjected to a testing or screening step. Alternatively, the pixels 2are removed from the recesses 5 of the substrate 6 and are placed on adifferent testing substrate, such as an aluminum substrate prior to thetesting or screening step.

[0026] In a third preferred aspect of the first embodiment, thesuspension of the binder and the starting powders is cast or injectedinto recesses or cavity portions 5 of the forming substrate or mold 6,respectively, such that a base portion 7 of the emissive host material 1connects the individual pixels 2, as illustrated in FIG. 4. Once thebinder is evaporated and the host material 1 is activated and sintered,it is removed from the forming substrate or mold 6 and placed with thebase portion 7 down on a testing substrate 8, such as an aluminumsubstrate, as illustrated in FIG. 5. With the pixels 2 being distal fromthe substrate 8, dopants different type and/or concentration are thenadded to the upward facing host material pixels 2 and reacted to formdoped pixels 2. The doped pixels 2 are then subjected to a testing orscreening step.

[0027] The second and third aspects describe forming the host materialby depositing a powder in a suspension containing a solvent and abinder. However, the binder containing suspension may be omitted, andthe starting carbonate powders may be dry pressed into the desired pixel2 shape using a dry pressing apparatus. Alternatively, the powders maybe dry sintered into the solid pixels 2 while located in the recesses orcavities 5.

[0028] If desired, the top portions of the pixels 2 may contain wells 9that are capable of receiving the dopant solution, as illustrated inFIGS. 6 and 7. The wells 9 may be formed by etching the top portion ofeach pixel 2, or by locating a protrusion in the substrate or mold 6recesses 5.

[0029] The dopants provided into the pixels can be reacted with thepixels using a number of different conditions. For example, the dopantscan be provided into activated and/or sintered emissive host materialpixels 2 and then reacted with the host material by applying thermal,infrared or microwave heating to the pixels. Alternatively, the dopantscan be provided into the host material prior to the activation and/orsintering steps. In this case, the dopants are reacted with the hostmaterial during the activation and/or sintering steps. Other usefulreaction techniques that can be used to simultaneously react thecomponents of interest will be apparent to one of skill in the art.

[0030] The dopants are preferably added to a single host material. Eachpixel of the host material contains a different dopant type and/orconcentration. By synthesizing an array of emissive materials in asingle host material, screening the array for materials having usefulproperties is more easily carried out. Preferred dopants includerubidium (Rb), cesium (Cs), iridium (Ir), scandium (Sc) and rare earthsfor the triple oxide emissive host material. The dopant concentrationpreferably comprises from about 5% to about 50% by weight of theemissive material.

[0031] The dopants are preferably ions that added to the host materialin the form of a precursor in a liquid solution. If desired, thesolution may contain counter ions such as nitrate, acetate, bromide,chlorate, chloride, salicylate, stearate, sulfate, or tartrate saltdissolved in a solvent comprising water, alcohol, or mixture of waterand alcohol. In one preferred aspect, oxide or carbonate compounds ofthe Rb, Cs, Ir, Sc and/or rare earth dopants are added to barium,strontium and carbonate host material powders in a slurry before theannealing step. The host material and dopant carbonate or oxide powdersare then co-sintered. In another preferred aspect, a solution containingpure metal dopant ions, such as Rb or Ir is provided to the sinteredhost material pixels.

[0032] In the delivery system of the first preferred embodiment, aprecisely quantified amount of each dopant is delivered into eachreaction region of the host material, such as a top surface or well 9 ofeach pixel 2. Any dopant delivery system which is capable of deliveringa precisely quantified amount of each dopant type and/or concentrationmay be used to deliver the dopants into the host material pixels 2. Theskilled artisan will appreciate that this may be accomplished using avariety of well-known delivery techniques, either alone or incombination with a variety of masking techniques.

[0033] Dopants can be deposited into the reaction regions of interestfrom a dispenser in the form of droplets by a variety of techniques wellknown in the art. These include a micropipetting apparatus or an ink-jetdispenser system, including a pulse pressure type dispenser system, abubble jet type dispenser system and a slit jet type dispenser system.Such dispenser systems can be manually or automatically (i.e., computer)controlled.

[0034] The dispenser system can be aligned with respect to theappropriate pixels 2 by a variety of alignment systems available in themicroelectronic device fabrication and combinatorial chemistry arts. Forexample, the reference points or alignment marks on the surface of thepixel array can be accurately identified by using capacitive, resistiveor optical sensors. Alternatively, an alignment system using a cameraand an image processor can be employed.

[0035] The pixels are doped by absorbing liquid media into the pores ofthe ceramic emissive host material. Capillary action drives the dopantinto the interstices between the ceramic grains. The solvent may beremoved during the activation, sintering or reacting step.

[0036] According to the second preferred embodiment of the presentinvention, an array of pixels of emissive materials having a differentcomposition and/or stoichiometry is deposited on a single substrate. Thecomposition and stoichiometry of each pixel 12 of a deposited emissivematerial 11 in the array may be varied by depositing the materials ofeach pixel 12 via several independent particulate injectors 13, asillustrated in FIG. 8. The injectors 13 are positioned over a largesubstrate, such as a conductive aluminum substrate 14. Each injector 13contains an emissive material precursor having a different compositionand/or stoichiometry. The injectors 13 provide the emissive material 11onto the substrate 14 to form the respective pixels 12. For example, theinjectors 13 may contain a dry metal oxide powder or a metal oxidepowder in a fluid suspension. The particulate injectors 13 may compriseink jet dispensers, micropipettes, or pneumatic slurry dispensers. Ifdesired, instead of being deposited on a flat substrate 14, the emissivematerial 11 may be deposited into recesses 5 of a grooved substrate 6illustrated in FIGS. 2 and 3. The deposited pixels 12 of the hostmaterial 11 are activated and/or sintered and then screened for usefulemissive properties, such as work function.

[0037] One preferred emissive material of the second preferredembodiment is barium tantalate which contains at least one of strontium,calcium and tungsten. This material has the formula (A_(1−x),Ca_(x))_(p) (Ta_(1−y), W_(y))_(q) O_(r+y), where A comprises one ofbarium or a combination of barium and strontium, p=2-6, q=2-6, r=4-12,0≦x<0.5, 0≦y<1. Tantalum has a +5 oxidation state, while tungsten has a+6 oxidation state. Therefore, the number of moles of oxygen will dependon the number of moles of tungsten in the emissive material.

[0038] Starting barium, tantalum, calcium and/or tungsten powders, suchas a BaCO₃ powder, a Ta₂O₅ powder and at least one of a CaCO₃ powder, aSrCO₃ powder and a WO₃ powder, containing different stoichiometriesand/or different elements are mixed and placed into individual injectors13. The powders may be provided in a suspension containing a solvent anda binder, if desired. The injectors 13 then form individual emissivematerial pixels 12 on the substrate 14. The pixels 12 are then sinteredto form a plurality of sintered pixels 12 having a unique compositionand/or stoichiometry on the substrate 14. Preferably, the sinteringtakes place by thermal, radiative or RF heating at a temperature ofabout 1300° C. to about 1500° C. for about 8 to 10 hours. However, otherappropriate sintering temperatures and durations may be used.

[0039] According to one preferred aspect of the second embodiment, eachpixel 12 of the emissive material contains a different value of p, q, r,x and y. After each pixel 12 is deposited, its emissive properties, suchas work function, are screened or tested. According to another preferredaspect of the second embodiment, each pixel 12 contains the same valueof p, q and r. Preferably, p=6, q=2 and r=11 in the (A_(1−x) Ca_(x))_(p)(Ta_(1−y) W_(y))_(q) O_(r+y) emissive material, such that each pixel ofthe emissive material has the following formula: (A_(1−x) Ca_(x))₆(Ta_(1−y) W_(y))₂ O_(11+y), where least one of x or y is greater thanzero, and the number of moles of oxygen varies between 11 and 12. Thus,the emissive mixture is a solid solution of (A_(1−x) Ca_(x))₆ Ta₂ O₁₁and (A_(1−x) Ca_(x))₆ W₂ O₁₂ when the emissive mixture containstungsten.

[0040] The emissive material having a 6:1 barium oxide to tantalumpentoxide molar ratio (i.e., a Ba/Ta ratio of 3) that contains calciumand/or tungsten has improved properties, such as a lower work function,compared to a similar emissive material that lacks calcium and/ortungsten, as well as compared to emissive mixtures having other bariumoxide to tantalum pentoxide molar ratios (such as 5:2 or 4:1 ratios,which correspond to Ba/Ta ratios of 1.25 or 2, respectively). Thus, eachpixel 12 of the emissive material preferably contains the same value ofp, q and r, but a different value of x and y. For example, the preferredcalcium amount, x, of the emissive material is greater than zero andless than 0.4, preferably 0.1≦x≦0.3, while the tungsten amount may bezero or greater than zero. The preferred tungsten amount is greater thanzero and less than 0.75, preferably 0.25≦x≦0.5, while the calcium amountmay be zero or greater than zero. Most preferably, the emissive mixturecontains non-zero amounts of calcium and tungsten. Therefore, each pixel12 has a different stoichiometry from the other pixels (i.e., adifferent value of x and/or y) or a different composition from the otherpixels (i.e., contains Ca or W that is not contained in other pixels).

[0041] Another preferred emissive material of the second embodimentcomprises a two-component yttrium oxide (Y₂O₃) and lanthanum oxide(La₂O₃) material 11. The emissive material 11 contains 0.5 to 80 molarpercent La₂O₃ and 99.5 to 20 molar percent Y₂O₃. Thus, each pixel 12 hasa unique stoichiometry by virtue of having a different La₂O₃:Y₂O₃ molarratio, which preferably ranges from 19:1 to 1:9.

[0042] In another preferred aspect of the second embodiment, theemissive material 11 further contains a third component in addition tothe La₂O₃ and Y₂O₃ components. Preferably, the third component containsbarium and oxygen, and may be selected from ((Ba,Sr)_(1−x) Ca_(x))_(p)(Ta_(1−y) W_(y))_(q) O_(r+y), (Ba, Sr, Ca)₃WO₆, BaO, BaO*CaO or thetriple oxide (BaO*CaO*SrO). The third component preferably comprises 70to 99 molar percent of the emissive material. Thus, each pixel 12 mayhave a different stoichiometry by virtue of having a differentLa₂O₃:Y₂O₃ molar ratio or and/or a different composition if the pixelcontains a different third component element. Furthermore, thestoichiometry of each pixel 12 may be varied by varying the amountand/or the stoichiometry of the barium oxide containing third componentof the emissive material, while the molar ratio of La₂O₃:Y₂O₃ is eitherthe same or different in each pixel 12.

[0043] Another preferred emissive material of the second embodimentcomprises a three component barium oxide, strontium oxide and calciumoxide (i.e., triple oxide) material. The emissive material of each pixel12 contains a different molar ratio of each oxide, which provides aunique pixel stoichiometry.

[0044] The pixels 2, 12 produced according to the first or secondpreferred embodiments are subsequently tested or screened for usefulelectron emissive properties. Preferably, the emissive propertydetermined is the work function of each pixel. Most preferably, the workfunction of the pixels 22 on a substrate 24 is determined by using aKelvin probe 23, as illustrated in FIG. 9. A Kelvin probe 23 is avibrating capacitor device which measures the work function (also termedsurface potential for non-metal materials) of a sample.

[0045] Any Kelvin probe may be used to measure the work function of theemissive material pixels 22. For example, a Kelvin probe 23 disclosed inBaikie et al., 69 Rev. Sci. Instr. 11, 3902 (1998), incorporated hereinby reference, may be used. This Kelvin probe 23 contains a referenceelectrode or probe tip 25 suspended above the pixels 22, thus creating asimple capacitor. The reference electrode 25 is vibrated in the vertical(i.e., z) direction by a voice coil element 26 containing a coil driver,magnets and springs. Control electronics 27, which include elements suchas a computer, digital to analog converter, a voltage detector and anoscillator, control the amplitude, height and frequency of oscillationof electrode 25 and the voice coil element 26. The control electronicsalso 27 provide a reverse bias or backing voltage, V_(b), between thepixels 22 and the reference electrode 25. The computer contains softwarewhich analyzes the work function measured on the plurality of pixels 22and provides a visual, electronic or printed output of the distributionof work functions for each pixel.

[0046] The Kelvin probe 23 operates as follows. When the referenceelectrode or tip 25 is contacted to the pixel 22, electrons flow fromthe electrode 25 having a lower work function, to the pixel 22 having ahigher work function, which produces a contact potential or voltage,V_(c) across the Kelvin capacitor 22/25. The variable reverse bias orbacking voltage, V_(b), is applied between the electrode 25 and thepixel 22. The value of V_(b) is continuously monitored. When value ofthe variable V_(b) equals to V_(c), no potential difference existsbetween the electrode 25 and the pixel 22. The work function of theemissive material of each pixel 22, φ, can then be determined from thefollowing equation: φ=eV_(b), where e=1.6×10⁻¹⁹ coulombs and V_(b)=V_(c)(i.e., the value of φ is determined when no potential difference betweenthe electrode 25 and the pixel 22 is detected).

[0047] The reference electrode or tip 25 can be used to sequentiallydetect the work function of every pixel 22 in the array by moving thesubstrate 24 below the electrode 26. The substrate 24 may be movedmanually or by using an XY stage driver 28 controlled by the controlelectronics 27. Alternatively, the electrode 25 may be moved relative toa stationary substrate.

[0048] Alternatively, the work functions of all pixels 22 in the arraymay be detected simultaneously by using a multitip Kelvin probe 33illustrated in FIG. 10. An example of a multitip Kelvin probe 33 isprovided in Baikie et al., 70 Rev. Sci. Instr. 3, 1842 (1999),incorporated herein by reference. The probe 33 contains a plurality ofreference cathodes or tips 35 which are aligned to the individual pixels32 on a substrate 34. Thus, the work function of each pixel 32 can besimultaneously determined by contacting each pixel 32 with therespective electrode 35 at the same time.

[0049] After each pixel is screened to determine a useful emissiveproperty, such as work function, the emissive materials which comprisethe pixels having the desired properties are provided for furthertesting. For example, the emissive materials may be coated ontofluorescent lamp cathodes and the lifetime, efficacy and/or cathode fallmeasurements may be carried out. The emissive materials, whosecomposition and stoichiometry were determined by the combinatorialchemistry methods may then be placed onto cathodes of commerciallyprovided devices, such as fluorescent lamps. Thus, the composition andthe stoichiometry of a pixel having a desired value of work function isselected, an electron emissive material having the selected compositionand stoichiometry is produced and placed onto an electrode of afluorescent lamp. The fluorescent lamp includes a shell, a phosphorformed on an inner surface of the shell, at least one electrode and theelectron emissive whose composition was determined by the above method.

[0050] The Kelvin probe 23, 33 may be used to measure the work functionof a plurality of emissive material pixels provided on a substrate by acombinatorial chemistry method. However, the Kelvin probe 23, 33 alsomay be used to measure the work function of a plurality of other organicor inorganic materials provided on a substrate by a combinatorialchemistry method, where the composition and/stoichiometry of each pixelof such material is different from the other pixels in the array.

[0051] Furthermore, while a Kelvin probe has been described as a devicewhich measures the work function, other devices may be used instead. Forexample, Auger spectroscopy, scanning tunneling microscopy,electrostatic force microscopy or atomic force microscopy may be used tomeasure the work function of the pixels. For example, the work functionof the pixels may be sequentially measured by scanning a cantileveredtip of an atomic force microscope over each pixel. Alternatively, thework function of the pixels may be simultaneously measured by providingone of a plurality of cantilevered tips of an atomic force microscopeover each respective pixel.

[0052] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the scope of the invention.

I claim:
 1. A method of making an electron emissive material,comprising: providing a plurality of pixels of the electron emissivematerial, each pixel having at least one different characteristic fromany other one of the plurality of pixels; and measuring at least oneproperty of each pixel.
 2. The method of claim 1, wherein the at leastone characteristic comprises at least one of stoichiometry andcomposition.
 3. The method of claim 2, wherein the at least one propertycomprises work function.
 4. The method of claim 3, wherein the step ofproviding comprises: providing a first dopant to a first pixel of a hostmaterial; providing a second dopant of a different type or concentrationthan the first dopant to a second pixel of the host material; reactingthe dopants with the host material to provide a first and a second pixelof an array of pixels of the electron emissive material.
 5. The methodof claim 4, wherein the step of providing further comprises: providing aplurality of dopants to the array of pixels of the host material,wherein a dopant provided to each pixel is of a different type orconcentration than a dopant provided to other pixels; and reacting thedopants with the host material to provide the array of pixels of theelectron emissive material.
 6. The method of claim 5, wherein: eachpixel of the host material comprises a triple oxide comprising bariumoxide, strontium oxide and calcium oxide; and the dopant comprises atleast one of rubidium, iridium, cesium, scandium and rare earths.
 7. Themethod of claim 6, further comprising: providing a differentconcentration of the dopants to each pixel of the host material having asame composition and stoichiometry; reacting the dopant and the hostmaterial to form the array of pixels where pixel each pixel has adifferent composition or stoichiometry from any other one of theplurality of pixels.
 8. The method of claim 7, further comprising:providing a liquid solution of the dopant in a solvent into wells in atop portion of the host material of each pixel; and heating the pixelarray to react the dopant and the host material.
 9. The method of claim8, further comprising: providing a block of a host material; and dicingthe block into a plurality of pixels of host material.
 10. The method ofclaim 8, further comprising: providing a suspension containing astarting metal oxide powder into a plurality of cavities on a firstsubstrate; and heating the suspension to provide the host material pixelarray, wherein each pixel is located in a respective cavity.
 11. Themethod of claim 10, further comprising: providing a host material baseconnecting the plurality of pixels located in the plurality of cavitieson the first substrate; removing the host material from the firstsubstrate; and placing the base onto a second substrate such that theplurality of pixels are distal from the second substrate.
 12. The methodof claim 3, wherein the step of providing comprises: providing a firstpixel of a host material; providing a second pixel of the host materialhaving a different composition or stoichiometry than the first pixel.13. The method of claim 12, wherein the step of providing furthercomprises providing an array of a plurality of emissive material pixels,each pixel having a different composition or stoichiometry than theother pixels of the array.
 14. The method of claim 13, furthercomprising: providing a particulate material having a differentcomposition or stoichiometry from a plurality of independent particulateinjectors; and heating the particulate material to provide the array ofemissive material pixels.
 15. The method of claim 14, wherein: theemissive material comprises (A_(1−x), Ca_(x))_(p) (Ta_(1−y), W_(y))_(q)O_(r+y), wherein A comprises one of barium or a combination of bariumand strontium, p=2-6; q=2-6; r=4-12; 0≦x<0.5; and 0≦y<1; and each pixelhas a different value of at least one of p, q, r, x and y from the otherpixels.
 16. The method of claim 15, wherein: the emissive materialcomprises (A_(1−x) Ca_(x))₆ (Ta_(1−y) W_(y))₂ O_(11+y), where least oneof x or y is greater than zero; each pixel has a different value of atleast one of x and y from the other pixels; 0≦x≦0.4; and 0≦y≦0.75. 17.The method of claim 15, wherein: the emissive material comprises Y₂O₃and La₂O₃; and each pixel contains a different molar ratio of Y₂O₃ toLa₂O₃.
 18. The method of claim 17, wherein the emissive material furthercomprises a material containing barium and oxygen.
 19. The method ofclaim 3, wherein the measurement comprises measuring the work functionof the emissive material using Auger spectroscopy, scanning tunnelingmicroscopy, electrostatic force microscopy or atomic force microscopy.20. The method of claim 3, wherein the measurement comprises measuringthe work function of the emissive material using a Kelvin probe.
 21. Anarray of an electron emissive material pixels made by the method ofclaim
 3. 22. The method of claim 3, further comprising: selecting thecomposition and the stoichiometry of a pixel having a desired value ofwork function; producing an electron emissive material having theselected composition and stoichiometry; and placing the electronemissive material onto an electrode of a fluorescent lamp.
 23. Afluorescent lamp, comprising: a shell; a phosphor formed on an innersurface of the shell; at least one electrode; and an electron emissivematerial whose composition was determined by the method of claim
 22. 24.A method of determining a work function of a plurality of pixels of anelectron emissive material, comprising: providing an array of pixels ofa first material on a first substrate, each pixel having at least onedifferent characteristic from any other one of the plurality of pixels;and measuring the work function of each pixel on the first substrateusing a work function measurement device.
 25. The method of claim 24,wherein: the first material comprises a ceramic electron emissivematerial; and the work function measurement device comprises a Kelvinprobe.
 26. The method of claim 25, wherein the at least onecharacteristic comprises composition or stoichiometry.
 27. The method ofclaim 26, wherein the electron emissive material is selected from agroup consisting of: (a) barium oxide, strontium oxide and calciumoxide; (b) yttrium oxide and lanthanum oxide; and (c) (A_(1−x), Ca_(x))₆(Ta_(1−y), W_(y))₂ O_(11+y), wherein A comprises one of barium or acombination of barium and strontium; 0≦x<0.5; 0≦y<1; and at least one ofx and y is greater than zero.
 28. The method of claim 25, furthercomprising: positioning a Kelvin probe tip over a first pixel; measuringthe work function of the first pixel; moving the first substraterelative to the Kelvin probe tip, such that the Kelvin probe tip ispositioned over a second pixel; and measuring the work function of thesecond pixel.
 29. The method of claim 25, further comprising:positioning a plurality of Kelvin probe tips over a plurality ofrespective pixels; and simultaneously measuring the work function of theplurality of pixels.
 30. The method of claim 24, wherein the workfunction measurement device comprises a scanning tunneling microscope.31. A Kelvin probe combinatorial testing system, comprising: a Kelvinprobe apparatus; a first substrate adapted to support a plurality ofpixels of a material to be tested, each pixel having at least onedifferent characteristic from any other one of the plurality of pixels;and a computer electrically connected to the Kelvin probe apparatuscontaining software which analyzes a work function measured on theplurality of pixels and which provides a visual, electronic or printedoutput of the work function of each pixel.
 32. The system of claim 31,further comprising a plurality of pixels of an electron emissivematerial to be tested, each pixel having at least one differentcharacteristic from any other one of the plurality of pixels, located onthe first substrate.
 33. The system of claim 31, further comprising adriver which is adapted to move the first substrate such that a tip ofthe Kelvin probe is positioned over a particular pixel.
 34. The systemof claim 31, further comprising a plurality of Kevin probe tipspositioned over a plurality of the first substrate locations adapted tosupport a pixel.